New Discoveries about the Immune System Impacts the Brain

Depression, autism and Alzheimer’s disease are all painfully prevalent brain disorders, but they have something else in common: they have been linked to inflammation in the brain. This essential immune response that helps us fight infections sometimes backfires—when prolonged, the action can damage tissue and negatively affect our mood and cognitive abilities. Think about the last time you had a cold: That lousy, drowsy feeling can be attributed to this effect.

Scientists are starting to understand how immune responses contribute to disorders of the brain and are beginning to test new drugs that target inflammation. Clinical trials assessing the effectiveness of anti-inflammatory drugs for depression and schizophrenia are already underway. Researchers have even tested an experimental vaccine that manipulates the immune system to lessen the fear response in rodents—a technique that could someday be used to treat and prevent post-traumatic stress disorder.

This year, scientists also discovered an unexpected line of communication between the immune system and the brain—a previously unseen network of lymphatic vessels in the meninges, the membranes surrounding the brain, that drains fluids from tissues and carries immune cells. Though abnormal immune activity had been associated with psychiatric conditions in the past, scientists believed the brain’s immune system to be separate from the body’s. This new finding suggests that malfunction in these vessels could contribute to a variety of neurological diseases with an immune component including Alzheimer’s and autism.

Not only were the connections between the brain and immune system further solidified by studies published in 2015, but the brain’s resident immune cells also emerged as a key player in both development and disease. Though microglia were long seen solely as the primary defenders of the brain, scientists have recently recognized their critical role in eliminating unwanted synapses, the connections between neurons, during early development. Research presented at this year’s Society for Neuroscience conference in Chicago revealed how defective pruning by microglia might lead to developmental disorders like schizophrenia and autism. Scientists also provided preliminary evidence that microglial activity later in life could play a role in neurodegenerative disorders such as Huntington’s disease and blocking this activity might reduce the loss of synapses seen in such diseases. These new revelations will likely lead to better treatment options for a variety of psychiatric and neurological conditions in the years to come.